1. Field of the Invention
This invention relates to a magnetic recording medium such as a magnetic tape, a magnetic card or a magnetic disk. More particularly, it relates to an improvement of a binder contained in a magnetic layer formed on a non-magnetic support.
2.Description of the Related Art
In general, a magnetic recording medium such as a magnetic tape or a floppy disk is prepared by coating a magnetic paint formed by dispersing a ferromagnetic powder together with an organic polymer as a binder in an organic solvent on a polymeric film or sheet.
As the organic polymer in the magnetic paint, a relatively hard resin such as polyvinyl chloride, polyvinyl butyral, nitrocellulose or acetylcellulose and a soft resin such as a polyester, a polyurethane or an acrylonitrile/butadiene copolymer are often used in combination. Moreover, to improve the durability of the coating, a polyisocyanate compound is often incorporated as a heat-curing component. Accordingly, the organic polymer used as the binder must have an appropriate compatibility with other polymer and an appropriate reactivity with the polyisocyanate, a good affinity with the magnetic powder and a function of dispersing the magnetic powder homogeneously in the paint.
Due to a recent increase in the demand for a higher recording density and a higher S/N ratio in the magnetic recording material, the particle size of the magnetic powder is made finer and the coercive force of the magnetic powder is increased, and therefore, the dispersing capacity of the binder is an important factor for dispersing the magnetic powder uniformly in the paint, forming a smooth magnetic layer having a high degree of filling, and improving the performances of the binder.
To improve the dispersion state of the magnetic powder, usually a method is adopted in which a surface active agent having a low molecular weight is used as the dispersant, but in view of the durability of the magnetic recording medium and a prevention of contamination of the head, the amount of the dispersant used is naturally limited, and to improve the reliability of the magnetic recording medium, the binder must have a dispersing capacity. Separately, a method in which a curable compound represented by a polyisocyanate compound is incorporated in a paint whereby the formed magnetic layer is crosslinked is customarily adopted for increasing the durability and reliability of the recording medium, especially in the field of a recording magnetic tape, and in this case, the binder must have an appropriate reactivity with the polyisocyanate. If this reactivity is too high, however, the pot life of the paint is short and the paint is disadvantageous from the economical viewpoint. If the reactivity is too low, the desired improved durability or traveling property cannot be obtained.
With a view to improving the binder, so as to cope with this increase of the performance of the magnetic recording medium, especially in connection with polyvinyl chloride (PVC) binders, the inventors investigated known PVC binders, and found the following. In a vinyl chloride/vinyl alcohol/vinyl acetate copolymer, the dispersing capacity for the magnetic powder increases with an increase of the ratio of the vinyl alcohol, but the degree of the improvement is low and the aid of a dispersant is necessary. Furthermore, as the proportion of the vinyl alcohol increases, the pot life becomes short and the moisture resistance of the coating is degraded, and furthermore, the rate of the thermal decomposition increases. A vinyl chloride/maleic acid/vinyl acetate copolymer is fully adsorbed in the magnetic powder and exerts an excellent dispersing capacity, but an increase of the viscosity or gelation readily occurs and the reaction with the polyisocyanate tends to occur locally.
As the means for solving the problems of typical PVC binders, there have been proposed a method in which parts of OH groups of a vinyl chloride/vinyl alcohol/vinyl acetate copolymer are substituted with an organic group having a hydrophilic group such as COOM, SO.sub.3 M, SO.sub.4 M or PO(OM).sub.2 (in which M represents hydrogen, an alkali metal or ammonium) (Japanese Unexamined Patent Publication No. 57-44227), a method in which a vinyl chloride/vinyl acetate copolymer having a hydrophilic group as mentioned above is saponified (Japanese Unexamined Patent Publication No. 58-114330, No. 61-57640 and No. 61-96515), and a method in which a vinyl chloride/vinyl alcohol/vinyl acetate copolymer having an ion-dissociative hydrophilic group introduced therein is used. According to these proposals, the dispersibility is improved but the level of improvement level is still too low, and since these methods include the saponification step the heat stability of the polymers is poor, and in magnetic recording media formed by using these polymers, there is a risk of a reduction of the reliability with time.
Furthermore, a method has been proposed in which a copolymer of a hydroxyl group-containing monomer such as hydroxylethyl (meth)acrylate or hydroxylpropyl (meth)acrylate with an ion-dissociative hydrophilic group-containing monomer is used so that deterioration of the polymer by the saponification is avoided (Japanese Unexamined Patent Publication No. 56-77930, No. 60-185226, No. 60-235814, No. 62-73416, No. 62-73417 and No. 62-134819). In this case, the dispersibility and heat stability are improved, compared with the case where the polymer comprising vinyl alcohol units are used as the hydroxyl group-containing groups, but the improvement is still unsatisfactory, and at the time of application, the pot life after the addition of the polyisocyanate compound is often too short.
A lubricant represented by a fatty acid is added to a magnetic paint to maintain a good traveling performance, but if the heretofore proposed vinyl chloride binders contain a hydroxyl group in an amount enough to impart a sufficient degree of crosslinking by the reaction with the polyisocyanate, as pointed out above, the problem arises of a reduction of the dispersion stability of the magnetic paint, and this tendency becomes conspicuous as the particle size of the magnetic powder is decreased.